The present invention relates to a method of manufacturing a mixed crystal epitaxial wafer which is suitable for use as a material for manufacturing light emitting diode (hereinafter referred to as "LED") and which is made of compound semiconductor containing elements belonging to the III and V groups in the periodic table. This type of material is referred to as III-V semiconductor material or compounds hereinafter.
LED's which emit red or green light have been manufactured generally by using wafers of III-V semiconductor material such as GaAS, GaP and the like. LED's which emit intermediate colors between red and green, such as orange or yellow, have been manufactured using a mixed crystal epitaxial wafer of GaAs.sub.1-x P.sub.x and the like. The use of mixed crystal epitaxial wafers provides the advantage that the forbidden energy gap, which determines the wavelength of the emitted light, can be varied by changing the mixed crystal ratio x.
In order to prevent crystal lattice dislocations between a monocrystalline substrate and the mixed crystal epitaxial layer during the growth of a mixed crystal epitaxial layer, a gradient layer is provided between the substrate and the epitaxial layer having a mixed crystal ratio which varies continuously in the thickness direction. For example, a mixed crystal epitaxial layer of GaAs.sub.0.35 P.sub.0.65 (mixed crystal ratio x=0.65) suitable for manufacturing orange LED (peak wavelength emitted light 360 mn.+-.10 nm) can be produced by epitaxially growing on a GaP monocrystalline substrate a layer portion in which the mixed crystal ratio x changes from 1 to 0.65 after which a layer having a constant value of x=0.65 is grown.
In this case, in order to keep various crystal defects, such as those referred to as pyramid, pit and void defects, as small as possible, it is necessary to maintain the epitaxial growth temperature, i.e., the temperature of the monocrystalline substrate, at a high temperature of around 900.degree. C. or more. This high temperature reduces the speed of epitaxial layer growth. Further, in the conventional method, the mixed crystal ratio in the gradient layer varies continuously. Therefore, a crystal lattice dislocation between the substrate (GaP) and the epitaxial layer (GaAs.sub.1-x P.sub.x, 1&gt;x&gt;0) cannot be removed sufficiently and thus the brightness (optical output) of a LED manufactured from such an epitaxial wafer is unsatisfactory.
A technique is known with which, during the growth of the gradient layer, the temperature of the monocrystalline substrate is varied in accordance with the value of x. However, with this technique, both the mixed crystal ratio, and hence the concentration of components contained in gas used for epitaxial growth, and the temperature of the monocrystalline substrate vary simultaneously and therefore an increase of crystal defect contained in the epitaxial layer being formed is unavoidable.